The present invention relates generally to techniques for recovering valuable chemical products. More particularly, the invention relates to highly efficient and economic processes for selectively recovering desired products from liquid mediums in which they are contained along with impurities, involving novel thermally-assisted chromatographic techniques.
As further background, the recovery and purification of acidic compounds such as carboxylic acids and other valuable chemical products from mediums has long been studied in an effort to discover efficient, cost-effective routes for their production. For example, carboxylic acids such as citric acid and lactic acid are manufactured by fermentation in large scale worldwide. Such fermentations provide fermentation broths from which the desired acid must be recovered and purified. Where high volume manufacture is involved, the importance of keeping recovery costs to a minimum cannot be overemphasized.
Recent recovery work has focused on the use of solid polymeric adsorbent materials to recover carboxylic acids from fermentation mediums. In this approach, the fermentation broth is passed over the adsorbent which adsorbs the carboxylic acid, and the carboxylic acid is desorbed in some fashion to provide product. Generally, a wide variety of adsorbents and adsorption/desorption schemes have been proposed.
For example, Kawabata et al., in U.S. Pat. No. 4,323,702, describe a process for recovering carboxylic acids with a material of which the main component is a polymeric adsorbent having a pyridine skeletal structure and a cross-linked structure. The carboxylic acid is adsorbed on the adsorbent, and then desorbed using a polar organic material such as an aliphatic alcohol, ketone or ester.
Kulprathipanja et al., in U.S. Pat. Nos. 4,720,579, 4,851,573, and 4,851,574, teach solid polymeric adsorbents including a neutral, noniogenic, macroreticular, water-insoluble cross-linked styrene-poly(vinyl)benzene, a cross-linked acrylic or styrene resin matrix having attached tertiary amine functional groups or pyridine functional groups, or a cross-linked acrylic or styrene resin matrix having attached aliphatic quaternary amine functional groups. In their work, Kulprathipanja et al. describe "pulse tests" conducted under isothermal conditions in which they identify acetone/water, sulfuric acid, and water as desorbents.
South African Patent Application No. 855155, filed Jul. 9, 1985, describes processes in which product acids were recovered from their aqueous solutions. In the adsorption step, the acid-containing solution was passed through a column containing an adsorber resin consisting of a vinylimidazole/methylene-bis-acrylamide polymer, a vinylpyridine/trimethylolpropane trimethacrylate/vinyltrimethylsilane polymer, a vinylimidazole/N-vinyl-N-methylacetamide/methylene-bis-acrylamide polymer, Amberlite IRA 35 (Rohm & Haas--acrylate/divinylbenzene based polymer containing dimethylamino groups), or Amberlite IRA 93 SP (Rohm & Haas) or Dowex MWA-1 or WGR-2 (Dow Chemical) (these latter three being styrene/divinylbenzene based polymers containing dimethylamino groups). To desorb the acid, water, usually at a temperature of 90.degree. C., was allowed to pass through the column. However, the single-pass elution process described involves an inefficient use of heat energy and does not substantially maximize the potential use of the resins to achieve highly concentrated desorbed solutions. Additionally, resins employed in this South African application are relatively thermally unstable and thus substantially degrade during desorption procedures employing hot water.
International Applications PCT/US92/02107 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16534) and PCT/US92/01986 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16490) both by Reilly Industries, Inc., disclose desorbing lactic and citric acid, respectively, from divinylbenzene crosslinked vinylpyridine or other resins using steam or hot water. The resins employed have advantageous adsorption/desorption capacities and are highly thermally stable under the described hot water desorption procedures. Nonetheless, improved processes would provide greater efficiency in the use of the resins and of heat applied to the desorption, and would readily provide desorbed solutions of even higher product concentration.
Still further processes for recovering acid products have been reported. For instance, U.S. Pat. No. 5,412,126 describes processes in which citric acid is adsorbed on a base resin and then stripped using an alkylamine. The free acid is then recovered by dewatering the material and driving the amine off with heat. U.S. Pat. No. 5,032,686 describes a process in which citric acid is separated from sugars using an acid resin, whereas U.S. Pat. No. 5,382,681 describes a process in which an impurity-containing citric acid solution is first treated with base to convert the citric acid to trisodium citrate, whereafter the basic medium is passed over a base resin to separate impurities.
In light of this and other background in the area, there remains a need for improved, effective processes for purifying and recovering carboxylic acids and other valuable products. The present invention addresses these needs.